A mobile machine, such as a wheel loader, a track-type-tractor, a motor grader, or a haul truck, includes a transmission that transfers rotational power from an engine of the machine to wheels or other traction devices. A typical transmission includes two directional clutches (e.g., a forward clutch and a reverse clutch) that are selectively engaged to alter a power flow path between the engine and the wheels. During forward travel, only the forward clutch should be engaged to transfer mechanical rotation from the engine through the wheels in a forward direction. During reverse travel, only the reverse clutch should be engaged to transfer the same mechanical rotation through the wheels in a reverse direction. During a directional shift change (i.e., when shifting between the forward and reverse directions), a first of the two directional clutches will release at about the same time as or before the second of the directional clutches engages.
If the directional shift change is attempted while the machine is moving, significant momentum aligned with the first travel direction must first be dissipated before the machine can begin accelerating in the second and opposite travel direction. This momentum is generally dissipated via friction material inside the engaging clutch. That is, the second clutch will generally slip until sufficient power has been dissipated to avoid shock-loading and damage to the remaining drivetrain components of the machine. During slipping, some of the friction material of the second clutch is worn away. In some situations, an operator of the machine may attempt to manually brake the machine during shifting between travel directions to reduce the amount of momentum that must be absorbed by the friction material of the second clutch. By doing so, the lives of the clutches may be prolonged and/or the directional shift may be completed more quickly and/or at higher speeds. It can be difficult, however, for the operator to use the right amount of braking and to properly time engagement/disengagement of the clutches to allow for smooth and efficient shifting without causing undue wear or damage of the drivetrain components.
An exemplary transmission system is disclosed in U.S. Pat. No. 8,880,303 of Ishikawa et al. that issued on Nov. 4, 2014 (“the '303 patent”). The transmission system of the '303 patent includes a vehicle control unit that detects a vehicle travel speed and prevents a shift change in a direction opposite to a vehicle traveling direction, as long as the vehicle travel speed is faster than a first speed. Specifically, when the vehicle control unit detects that a user has selected a shift range in a direction opposite to the vehicle traveling direction, while the vehicle travel speed is faster than the first speed, the vehicle control unit carries out a compulsory deceleration via hydraulic brakes to stop the vehicle instead of permitting the shift change. The shift change is only then permitted after the vehicle is stopped and the engine of the vehicle is in an idle state. Since the vehicle control unit ignores an erroneous shift change in a direction opposite to the vehicle traveling direction during high speed travel, a breakdown of the vehicle is prevented.
Although the strategy employed by the vehicle control unit of the '303 patent may have some effect on vehicle component life, the focus of the strategy is accommodating operator error. In particular, the vehicle control unit may do little to improve shift quality during a desired high-speed shift. In addition, the vehicle control unit may not be applicable to situations where the operator has not made an error in requesting a directional shift change.
The disclosed transmission system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.